The use of volume ventilators in clinical medicine is well established and a number of these devices, to one degree or another, employ fluidic control elements. However, many of the known devices are large and cumbersome. Some require electric components, thus creating a safety hazard when used in an operating room where flammable gases are present. Others require relatively large power sources whether they be electrical of fluidic sources. Still others suffer the disadvantage that they must be sterilized after each use.
Warren U.S. Pat. No. 3,292,623 is exemplary of a prior art ventilator employing a fluidic element. However, the patented device has the disadvantage that the "power" is always on and thus much oxygen is wasted. Furthermore, it takes a relatively large negative inspiratory pressure to operate the controlling flipflop since this flipflop is actuated through its output port, and the expiratory impedance is clinically undesirable. Burns U.S. Pat. No. 2,280,832 shows another fluidic ventilator which also has an undesirably high expiratory impedance. Furthermore, contamination of this patented device is inevitable.
More recently, fluidic ventilators have been developed which provide more selectively in the operational mode. Wolter, et al., U.S. Pat. No. 3,736,949 is typical of these devices. However, insofar as is known, no fluidic ventilator has been developed which can operate in the assist, control, or intermittent mandatory ventilation mode, that is provided with sigh control, and can be operated as an intermittent positive pressure breathing device or a positive end expiratory pressure device.
While volume cycled ventilators are known in the art, most of these devices determine tidal volume by calibrating for a certain flow rate and then timing the flow over an interval of time. While this method is satisfactory as long as the flow rate remains constant, if provides an erroneous indication of tidal volume if, for example, the pressure of the source should vary.